I'm sure this has happened to everyone. You start to fill a form, change your mind without having submitted the form and close the window. Then you get some kind of email communication from the site. Either a cookie cutter mail about having left items in their shopping cart or a more personal mail asking if you need help or have a problem.

How can they get hold of your email when you A) aren't registered at the site and B) never submitted any part of the form?

I've been thinking it's cookies, ad services that share their knowledge about you with the site, but I'm not so sure anymore.

One possibility is that you entered your email before closing the form page, and the form had already captured the entered text with Ajax even though you didn't submit anything (you could go back to the site and check your HTTP traffic while typing in the form).

Mobile phone companies may sell at least their customers' mobile numbers to paying websites: https://pcforalla.idg.se/2.1054/1.175582 (in Swedish). If you have an email account with the same mobile company maybe they sell that too?

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ad services that share their knowledge about you with the site

The ad service would have to know your email to give it to sites. I guess Google could technically do that with GMail addresses, but probably they'd rather stay as the middleman and sell ad space in GMail.

One possibility is that you entered your email before closing the form page, and the form had already captured the entered text with Ajax even though you didn't submit anything (you could go back to the site and check your HTTP traffic while typing in the form).

Yes, it was something like that I was looking for, but didn't know if it was possible. And yes, I had entered my email but never submitted the form. It used to be that nothing was "transmitted" unless the form was submitted, but not so anymore I guess.

Thanks!

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The ad service would have to know your email to give it to sites. I guess Google could technically do that with GMail addresses, but probably they'd rather stay as the middleman and sell ad space in GMail.

I think they can get that when you fill a form... but I don't know how they do it.

I'll stop filling forms until I'm a 100 % sure I'll follow through. I already use fake data (except for country) if I only want to know the shipping cost and need to enter something for that reason. Haven't encountered a single site that isn't happy with example.com or mailinator.com.